Front-end processing or front end of line (FEOL) refers to the formation of semiconductor devices, such as transistors or capacitors, on or within a semiconductor substrate. The semiconductor devices are manufactured by depositing films, patterning these films and etching them in order to create the desired semiconductor device. Other steps may be additionally used such as annealing, doping, cleaning or polishing, for example.
Once the various semiconductor devices have been created they must be interconnected to form the desired electrical circuits. The back end of line (BEOL) involves creating conductive interconnecting wires that are isolated by insulating dielectrics. The insulating material was traditionally a form of SiO2 or a silicate glass but recent changes in technology requires new materials with a low dielectric constant (low-k). These new dielectric materials include SiOC, for example, and have dielectric constants of around 2.7 (compared to 3.9 for SiO2), although materials with constants as low as 2.5 or even lower (ultra low-k) are currently being used.
Historically, only a few level of interconnect layers have been necessary to electrically integrate the semiconductor devices. The interconnect layers have been isolated by insulating material and connected by connections called vias. Aluminum was the preferred material. Aluminum is still used in some applications today.
More recently, as the number of interconnect levels for logic devices has substantially increased due to the large number of semiconductor devices that are now interconnected in a modern integrated circuits the timing delay in the wiring has become significant prompting a change in wiring material from aluminum to copper and from the silicon dioxides to the new low-k/ultra low-k materials. This performance enhancement also comes at a reduced cost via damascene processing that eliminates processing steps.
Modern integrated circuit manufacturing technology uses a process called damascene processing. A damascene process deposits the dielectric material first as a blanket film, and is patterned and etched leaving holes or trenches. In “single damascene” processing, copper is then deposited in the holes or trenches surrounded by a thin barrier film resulting in filled vias or wire “lines” respectively. In “dual damascene” technology, both the trench and via are fabricated before the deposition of copper resulting in formation of both the via and line simultaneously, further reducing the number of processing steps.